The discussion throughout this specification comes about due to the realization of the inventor and/or the identification of certain related art problems by the inventor. Moreover, any discussion of material such as documents, devices, acts or knowledge in this specification is included to explain the context of the invention in terms of the inventor's knowledge and experience and, accordingly, any such discussion should not be taken as an admission that any of the material forms part of the prior art base or the common general knowledge in the relevant art in Australia, or elsewhere.
Polymeric materials incorporate a wide range of polymers including natural rubber co-polymers and synthetic rubbers such as SBR (styrene butadiene rubber) and butadiene rubber, nitride rubber, isoprene rubber, neoprene rubber and polysulphide rubber. A large variety of other organic and inorganic chemicals are also added to polymeric products such as tyres and rubber conveyor belts including vulcanizing agents, accelerators, retardants, pigments, fillers, reinforcing agents, softeners, anti-oxidants and desiccants.
The pulverization extrusion process described herein can be used in the effective reprocessing of the said waste polymeric materials, whereby the realization by the inventor enables specific utilization of extrusion in relation to the claims of this invention to precisely tailor the resultant extruded particulate polymeric material in having a more useful particulate size and associated surface area to suit the requirements of further downstream value-add processes for enabling the increased usage in industry of the said waste polymeric materials.
Pulverizing by Extrusion
Before the waste polymeric material can become more advantageous for downstream utilization in industry, it must be reduced to small sized particles by methods such as pulverizing. Pulverization techniques for polymers typically rely upon screw extruders imposing compressive shear on the polymer at specific temperatures depending on the polymer.
Based on this methodology, the solid state shear extrusion pulverization of waste polymeric material using an extruder that can achieve pulverization has been proposed by the present invention, the said invention also more efficiently and advantageously causing simultaneous resultant deformation and surface devulcanization of the said waste polymeric material.
One of the main problems associated with pulverization using an extruder is controlling the large amounts of heat generated due to the compression and shearing actions in the pulverization zone.
Further, there is a need for optimizing the pulverization extruder and pulverization extrusion process that can provide product tailored to suit the requirements of downstream industrial processes.
Extrusion Generally
It is known that extruders squeeze a feed material, applying pressure until it is ejected from the extruder. There are many different variations of extrusion equipment but most typically they include a hopper for holding and feeding raw material, and a hydraulically or mechanically driven means for applying pressure. Most extruders include a die for shaping the extruded end product.
The means for applying pressure typically include a single or twin screw auger powered by an electric motor, or a ram driven by hydraulic pressure. Screws commonly used in extruders include single flight metering screws, single barrier screws, double barrier screws, variable pitch flights, multi-start flights, slotted flights and two stage screws such as for vented extruders.
Extrusion processes are usually described as either ‘hot’, ‘warm’ or ‘cold’:                Hot extrusion is done at an elevated temperature to keep the material from work hardening and to make it easier to push the material, particularly if it is passed through a die.        Warm extrusion is done above room temperature, but below the recrystallization temperature of the material. It is often used to achieve a balance between required forces, ductility and final extrusion properties.        Cold extrusion is carried out at or near room temperature. Cold extrusion has certain advantages over hot extrusion including lack of material oxidation, more precise tolerances and good surface finish.        
Vulcanization and Devulcanization
Vulcanization is the thermo-chemical process that incorporates sulphur into polymers in order to provide properties that are desired in manufactured polymer products. It is a chemical process for converting rubber or related polymers into more durable materials via the addition of sulphur. It is particularly extensively used in the production of tyres and rubber conveyor belts.
Typically, in order to be able to reuse polymer material from manufactured polymer products, it is first necessary to reclaim the polymer. The principal step of reclamation is devulcanization.
Devulcanization is the process of cleaving the intermolecular bonds of the polymer such as the sulphur-sulphur bonds with further shortening of the polymer chains. This is typically done by chemical, ultrasonic, microwave or biological processing, whist the present invention achieves this mechanically with control of the thermal environment.
The resultant devulcanization of the surface area of the pulverized waste polymer material by the present invention is achieved by utilizing the herein described pulverization extruder for selective cleavage of the sulphur-sulphur bonds by more economical mechanical means of devulcanization by an extruder through intense mechanical working of waste polymeric materials whilst simultaneously subjecting the said waste polymeric materials to variations in temperature ranging from hot extrusion to cold extrusion in the one process run in order to avoid the softening point of the specific polymeric material being batch processed, where the said batch material's softening point is the temperature at which a material softens beyond the softness determined by, for example, the Vicat method (ASTM-D1525 or ISO 306) in order to achieve a solid state polymeric crumb where sulphur-sulphur bonds are cut on the surface area of the said polymeric crumb without total desulphurization of the resultant crumb.
For illustration purposes of the preferred embodiment of the present invention in relation to the devulcanization by the present invention of for example rubber polymeric material reinforced with fiber, the temperature at the upper limit of the hot extrusion procedure reaches a maximum temperature of 300° C. and is then rapidly dropped to room temperature during the same process run to prevent the degradation of the rubber as illustrated in the fiber reinforced rubber degradation experimentation graph shown in FIG. 6, as well as optimize its solid state surface area characteristics of producing a larger surface area relative to the size of the resultant rubber crumb via thermal shock.
The achieved surface area devulcanization of the resultant rubber crumb is demonstrated in the Fourier transform infrared spectroscopy (FTIR) comparative analysis graph shown in FIG. 7.
The FTIR graph shown in FIG. 7 represents the absorbance at specific wavenumbers of the devulcanized rubber crumb output post the pulverization process relative to the control black line absorbance magnitude of vulcanized rubber prior to the pulverization process, with the most significant information obtained from the peaks at 1539 cm−1 and 1648 cm−1, respectively.
The specific devulcanization process of the present invention enables the product to be more effectually used in downstream processing for industrial applications as the resultant crumb, such as for example rubber crumb, is not degraded due to the typical continuous high temperature generated by pulverization extruders.
Further, as the said resultant crumb of the present invention is not totally desulphurized, the said crumb's usage for downstream industrial vulcanization/re-vulcanization processes produce greater vulcanized products with greater tensile strength which is useful in industry. This phenomenon is contrary to previous beliefs that total desulphurization of vulcanized materials will necessarily result in a more useful product for industrial uses.
The study ‘Evaluation of Waste Tyre Devulcanization Technologies—Report for the Integrated Waste Management Board of the State of California’ produced under contract by CalRecovery, Inc. in December 2004 for the Unites States of America's State of California Governor Arnold Schwarzenegger and Secretary, California Environmental Protection Agency Alan C. Lloyd, Ph.D. demonstrated that in relation to devulcanized grades of products “[t]here are no industry or common product specifications and grade definitions. Accordingly, there is no consensus on the devulcanization product grades. The companies promoting and developing devulcanization programs use a mechanism that allows a degree of understanding of the material in question.” Further, it was confirmed that “[p]rocess operating conditions such as temperature, residence time, and other process variables can change the devulcanized rubber characteristics.”
Further, a study by E. Alan McCaslin titled ‘Computer modeling of rubber vulcanization’ published in Rubber World, December 2007 confirms that “[e]ven when optimum cure times have been determined, they are subject to change from intentional compound formula modifications or unintended variations in component materials.” Specifically, in relation to vulcanization theory, it is confirmed that “[t]here are two predominant systems used for vulcanizing rubber, including organic peroxide and accelerated sulfur.” Further, it is disclosed that “it is not possible to express the degree of vulcanization of accelerated sulfur-cured rubbers in terms of percentage of remaining peroxide”.
Furthermore, a study by Hamid Yazdani et al titled ‘Devulcanization of Waste Tires Using a Twin-Screw Extruder: The Effects of Processing Conditions’ (Journal of Vinyl & Additive Technology, 17:64-69, 2011) has demonstrated that there is no direct correlation between a higher percentage of devulcanization and increased tensile strength, where
      Percentage    ⁢                  ⁢    of    ⁢                  ⁢    Devulcanisation    =                              v          ⁢                                          ⁢          1                -                  v          ⁢                                          ⁢          2                            v        ⁢                                  ⁢        1              ×    100  and where v1 and v2 are the cross-link density of the samples before and after the devulcanization, respectively.
Respective samples % of devulcanisation tableSamples123456789101112Devulcanisa-657570886560658375758085tion %
A graph showing the results set forth in the above table is shown in FIG. 8.
The study also demonstrated that although a combination of screw speed and barrel temperature could break the cross-links selectively, because the methodology's constant barrel temperature measured at the head of the extruder and no control over the internal temperature of the extruder screw, severe degradation could occur.